In Raman spectroscopy, a sample comprised of one or more molecules can scatter or absorb a photon incident upon the sample. The molecule-photon interaction can temporarily change the energy state of a molecule within the sample, i.e., the energy state of a molecule can be changed from an initial state to an excited state through a dipole-allowed transition. After a short period of time, typically less than about 10−14 seconds, the molecule can relax from its excited state with the concomitant emission of a new photon. The energy of emitted photons as well as photons scattered by the sample can be classified into one of three categories.
First, an emitted photon having less energy (i.e., lower frequency) than the incident photon is referred to as a “Stokes” emission. Stokes emissions occur when a molecule absorbs incident photon energy and relaxes into an excited rotational and/or vibrational state. Each molecular species in a sample can generate a characteristic set of Stokes emissions, the intensity of which is proportional to the density in the sample of the molecular species.
Second, an emitted photon having more energy (i.e., higher frequency) than the incident photon is called an “Anti-Stokes” emission. Anti-Stokes emissions can occur when an incident photon interacts with a molecule already in an excited state. During the molecule-photon interaction, the molecule can decay from the excited state to a lower energy state. The anti-Stokes photon will be emitted with the energy of the incident photon plus the difference in energy between the molecule's excited state and its lower energy state. As with Stokes emissions, each molecular species in a sample can generate a characteristic set of anti-Stokes emissions, the intensity of which is proportional to the density in the sample of the molecular species. Stokes and Anti-Stokes emissions (collectively Raman emissions) can provide quantitative information about the molecular species contributing to the scattering process.
Third, an elastically scattered photon has the same energy as the incident photon. Sample molecules that contribute to elastic scattering return to their initial energy state. Typically, the intensity of elastic (or Rayleigh) scattered photons dominates the scattering/emission spectra. different filter arrangements have been used to remove or reduce the intensity of the Rayleigh scattered photons. For example, triple monochromators, edge filters, and notch filters have been used. These filters arrangements are expensive and/or result in the attenuation of at least a portion of the Raman spectrum.